Process for the production of high-alpha cellulose fiber for the manufacture of cellulose derivatives



Patented Dec. 31, 1929 UNITED STATES PATENT OFFICE GEORGE AIRIGHTER, OFBERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, I OF BERLIN, NEWHAMPSHIRE, A CORPORATION OF MAINE PROCESS FOR THE PRODUCTION OFHIGH-ALPHA CELLULOSE FIBER FOR THE MANU- FACT'URE OF CELLULOSEDERIVATIVES silk, and as anew rag substitute in the manufacture of highgrade writing papers.

In certain other applications, I have disclosed processes which involvethe treatment of raw or unbleached pulp with an alkaline liquor toremove non-alpha cellulose constit uents contained therein, followed bya bleaching operation, for the production of a white high alphacellulose fiber. The alkaline treatment, i. e., the treatment of'the rawpulp with an alkaline liquor, moreover. may, in certain instances, besupplemented by a pretreatment of such pulp with a chlorinating oroxidizing liquor, thereby effecting the removal of certain non-alphacellulose constituents and conditioning such pulp for subsequentalkaline treatment by rendering such non-alpha cellulose constituentsmore susceptible to removal by the alkaline treatment. The alkalinetreatment may be carried out, as desired, either at an elevatedtemperature, in which case a relatively dilute solution of alkalinepurifying compounds isaemployed,'or at "a relatively low temperature,'(say, room temperature) in which case the concentration of alkalinecompounds in the treating liquor is relatively high.

In either case, however, whether the treatment be effected at relativelyhigh or at rel- 40 atively low temperature, the alkaline treatment,which efi'ectsarefinement or purification of the fiber, is followed by ableaching operation (and then a superbleaching operation, if desired)which, unless carried out under cer- 4 tain conditions andcarefullycontrolled, mayatfect the alpha cellulose content in thepurified fiber secured by previous treatment. Even when-the bleachingoperation is carried out under the desiredconditions, and is controlled,however, a certain small amount.

16, 1925." Serial No. 75,888.

terial or upon the extent of purification or refinement effected by thetreatments precedent to-the bleaching operation, or, in other words,upon the characteristics f the fiber before the bleaching operation iseffected.

Thus a less purified fiber may require more than the moderate amount ofbleach required in the bleaching operation of a highly purie fied fiberto yield the white color of the procluct desired. In such case, however,the alpha cellulose content, the copper number, and the generalpapermaking characteristics of 'the final product are affected. Thus there- -7 duction in the alpha cellulosecontentand the increase in coppernumber (owing to increased oxycellulose content of the product)primarily signifies that the fiber has been impaired in value and isless adapted for conversion into cellulose derivatives, as, for example,into viscose, and that the generalpaper-making characteristics ofthe'pulp have also been impaired.

With 'the foregoing in view, it might be inferred that a modifiedprocedure which com-f prises the purification of a bleached pulp, ratherthan an unbleached pulp, by treatment with an alkaline liquor to producea white high alpha cellulose fiber having the characteristics desired,should be adopted. I have found, however, that if a bleached pulp istreated with an alkaline liquor, it is impossible to obtain a fiberwhich has high strength, high tear test, and the desired alpha cellulosecontent. For instance, in working with a bleached sulphite pulpcontaining approximately 85% alpha cellulose and having a color of to102, I found that by an alkaline treatment of such pulp with a liq- 95nor containing about 6% to 8% caustic soda, based on pulp, at F. to 212F., for a suflicient length of time i. e., until an equilibrium in thetreatmenthad been reached, a product was obtained which had an alphacellulose content of about 91% and possessedbe drawn from this result isthat a bleached pulp is very susceptible to injury by an alkalineliquor, and results in a drastic reaction upon the fiber and in amaterial reduction of its strength and tear test. Principally becauseofthis fact, the process which I employ for the manufacture of a whitehigh alpha cellulose comprehends the alkaline treatment of an unbleachedpulp rather than bleached pulp, and a bleaching operation which issubsequent to the purifying alkaline treatment.

I have discovered, however, that the alpha cellulose content and thequalities desired in the white high alpha cellulose fiber produced bythe treatment of an unbleached pulp may be improved or enhanced by analkaline treatment: that is to saythat, although the alkaline treatmentof a bleached, raw pulp is accompanied by injurious effects on thefiber, a white, purified or high alpha cellulose stock derived from anunbleached raw stock which has been subjected to an alkaline treatmentand to a subsequent bleaching operation, may be improved in quality bysuit-able alkaline treatment. The final fiber thus produced has a veryhigh alpha cellulose content, and a very low copper number, possessessuperior papermaking qualities, and is hence particularly adapted forconversion into cellulose esters and other cellulose derivatives, and isalso suitable for use as a new rag substitute in the manufacture of highgrade bond and ledger papers. The production of the fiber herein desiredmay be *eifected from raw or unbleached pulp,

by a process comprising various combinations or sequences of treatments.Thus the initial raw stock may,-after a preliminary conditioningtreatment with an oxidizing or chlorinating liquor, be subjected to analkaline treatment at an elevated temperature, then bleached, andfinally treated with a hot alkaline liquor to yield the desired product.

Another sequence may be the initial conditioning of the unbleached pulp,followed successively by an alkaline treatment at an elevatedtemperature, a bleaching operation, and an alkaline treatment atrelatively low temperature. A further sequence may comprise the initialconditioning treatment of the unbleached raw stock, successivelyfollowed by a cold alkaline treatment, a bleaching operation, and a hotalkaline treatment, i. e., a treatment at relatively elevatedtemperatures. Or the sequence of treatments may be the conditioning ofan unbleached stock, followed by a treatment with an alkaline liquor atrelatively low temperatures e., a cold treatment), a bleachingtreatment, and a treatment with an alkaline liquor at relatively lowtemperature.

In eachsequence of treatments, the initial or pretreatment of the rawpulp with a chlorinating or oxidizing liquor which conditions the fiberfor purification with an alkaline liquor, may or may not be employed,depending upon the quality or type of unbleached fiber utilized as a rawmaterial. Thus in certain instances, when the raw pulp is of arefractory nature or contains a high percentage of ligneous matter,less-resistant celluloses, and other non-alpha cellulose constituents,as a result of the undercooking of the wood chips or other rawcellulosic material, or because of the quality of such raw material,such'pre-treatment of the raw pulp is preferably employed. In suchinstances,

the unbleached stock is treated at about room temperature at a totalstock density of about 5%, in a liquor containing about 2% chlorine,

or about 2% to 5% lime bleach, based on the weight of dry fiber. Thepre-tr'eatment requires a relatively short period of time (about twentyminutes to one hour) and is in no sense a bleaching operation, for thepulp is not whitened thereby. The liquor conditions the pulp forsubsequent alkaline treatment, resulting in the solution of certainnon-alpha cellulose impurities, or the formation of reaction productswhich aresubsequently dissolved during the treatment with an alkalineliquor. The stock is then washed free of the treating liquor and may bepurified by any one of a series of treatments which will presently bedescribed as specific examples.

Emample N0. 1.H0t alkaline treatment- Bleaching z-reazment-H0taU-t'alinc treafment Unbleached sulphite stock (pretreated, if desired)is treated at a consistency of about 10% in an alkaline liquorcontaining about 6% caustic soda, based on the weight of stock, or asuitable amount of other equivalent alkaline purifying agent, such as"sodium sulphide, at an elevated temperature (180 F. to 212 F.) for aboutsix hours. The alkaline liquor reacts with the non-alpha celluloseconstituents contained in the pulp, forming soluble reaction productsand thereby purifying the pulp and raising its alpha cellulose content.The purified pulp is washed free of alkaline liquor and entrainedproducts of reaction, and is then bleached at a stock consistency'ofabout 10% with about 5% to 0% lime or sodium bleach, resulting in a purewhite product containing 94% alpha cellulose and possessing a coppernumber of about 2.2. This product is high alpha cellulose fiber. Thishigh alpha cellulose fiber is retreated at a total stockconsistency ofabout 10% and at an elevated temperature (about 180 F. to 212 F.), witha relatively dilute alkaline liquor containing, say, about 1% to 4%caustic soda, based on the dry weight of 5 pulp. I

The treatment is continued for a sulficient period of time to yield afinal product having the desired characteristics, or until theoxycelluloses formed in the bleaching operation have been removed andthe copper number sufficiently lowered and its alpha cellulose contentincreased thereby. This treatment may require three to four hours,depending upon the characteristics of the bleached alpha stockundergoing treatment. The alkaline treated bleached alpha fiber iswashed and is then preferably neutralized in the cold to slight aciditywith a weak acid, such as, for example, sulphurous acid solution,resulting in a very white product having an alpha cellulose content ofabout 95% to 96% and a color of about 100 to 102. The copper number,moreover, has been reduced, testing about 1 to 1.5, and the pentosancontent has also been somewhat lowered by the final alkaline treatment.The final fiber is of extremely soft and velvety texture, closelysimulating new rag stock, and yields solutions of lower viscosity thanthe bleached and partially 3o purified fiber from which it is prepared,and of materially lower viscosity than the usual or standard cellulosicfiber, such as cotton, sulphite, soda, or kraft. Such change inviscosity is very desirable when solutions of cellulose or cellulosederivatives are employed for purposes where a high concentration and lowviscosity of solution is desired, and where the concentration to beobtained is limited by the viscosity of solution thereby produced. Thesefavorable changes in characteristics of bleached alpha fiber have beeneffected with a yield of about 95% to 98% of the bleached alpha fiberand without adversely affecting its strength and tear test, these valuesremainof the bleached alpha fiber prior to the alkaline treatment.

E mample 1V0. 2. H0t alkaline treatment Bleaeh00lt alkaline treatmentThe procedure in this example is similar to that followed in Example 1,with the exception of the final alkaline treatment, which is effectedwith a stronger alkaline liquor at relatively low temperature, say,approximatelv room temperature. Thus, by treating a bleached alphacellulose fiber at a total stock concentration of about 5% in a liquorcontaining about 25% to caustic soda based on the dry weight of fiber(corresponding to a solution containing 1.25% to 2.50% caustic sodaequivalent), for about one hour, a fiber is produced havingcharacteristics approximating those possessed by the fiber produced 65in accordance with the treatment outlined in ing about equal to thecorresponding values Example 1. The pentosan content, moreover, isreduced to a somewhat greater extent by the treatment specified in thepresent example.

E a'ample 1V 0. 3.U0l(l alkaline treat-ment- BleackH0t alkaltaetreatment Unbleached sulphite fiber (conditioned, if desired) is treatedwith a relatively concentrated alkaline liquor at relatively lowtemperature. This treatment is carried out, for example, at a 5% stockconcentration, in a liquor containing about 50% to 100 caustic soda,based on the dry weight of fiber. At a treating temperature of about 300., the treatment requires about one hour. The purified stock producedby such treatment contains about 94% alpha cellulose and possesses thedesired papermaking properties. The stock is then bleached with theusual moderate amount of bleach, then washed, and is finally treated atan elevated temperature with a dilute alkaline liquor under theconditions of final alkaline treatment set forth in Example 1. Theproduct obtained is substantially similar to the products produced inthe other example.

E sample N 0. 4.00lcl alkaline treatment- Bleach-U0lal alkalinetreatment In this example, the sequence of treatments includes no hotalkaline treatment of the fiber. Thus, the first alkaline treatment iseffected at relatively low temperature, under similar conditions to thefirst alkaline treatment specified in Example 3. The purified fiber isthen bleached and finally treated With a relatively strong alkalineliquor, under the condition of final alkaline treatment set forth inExample 2, and resulting in a similar product.

The raw material employed in the examples herein given, was a raw orunbleached sulphite pulp. Various other pulps, however, such as kraft,soda pulp, or pulps produced by various modifications of the sulphiteprocess, may be treated by the process herein set forth. Moreover, theconditions of treatment are given by Way ofillustration only, and aresubject to modification and variation. Thus, the bleaching treatment may'be effected in an alkaline bleach liquor, and a superbleaching of thefinal product herein produced with a chlorine solution may be practiced,if desired. In the treatment of pulps having different characteristics,the concentration of alkaline purifying compounds, bleach, and otherreagents used in the various treating steps may be modified to producethe results desired. For example, in certain cases the last alkalinetreatment may be effected on the purified pulp Which has been preparedunder certain modified conditions, so thatas a consequence it containsonly about 9.2% alpha cellulose, as compared to a usual alpha cellulosecontent of about 94%. In other words, the first alka line treatment maybe carried out in a liquor of a strength lower than usual, thusresulting in a less effective purification of the pulp,- the increase inalpha cellulose content of the pulp to a product having the desiredcharacteristics being eifected in the final alkaline treatment on a pulpwhich has been previously purified to an alpha cellulose content of atleast 90%. v The various treatments herein proposed are all preferablycontrolled to yield a final product having approximately similarcharacteristics, with a yield of such final product from the initial rawpulp as high as is consistent with optimum characteristics.

In the initial and final alkaline treatment, the caustic soda which isemployed as the alkaline purifying agent may be replaced in part or inentirety by an equivalent amount of one or a mixture of other suitablepurifying reagents, such as sodium carbonate, lime (either' dolomite orcalcite),'or sodium sulphide. However, in an alkaline treatment which iscarried out at relatively low temperature, the caustic soda may bereplacedonly by strongly alkaline agents such as sodium sulphide, as Ihave been unsuccessful in effecting the desired results under suchconditions by the use of sodium carbonate or lime.

Having thus described this invention, it should be obvious that itissusceptible of various changes and modifications without departing fromits spirit or scope. Hence I do not wish to limit myself to the specificexamples herein given.

What I claim is: I

1. A process for the production of a white high alpha cellulose fiberhaving good papermaking qualities and adapted for conversion intocellulose derivatives, which comprises treating raw pulp with analkaline liquor to remove non-alpha cellulose constituents containedtherein, thereby augmenting its alpha cellulose content to produce ahigh alpha cellulose fiber; bleaching said fiber; and then treating saidbleached fiber with an alkaline liquor to remove non-alpha celluloseconstituents contained therein due to said bleaching operation, therebyfurther augmenting its alpha cellulose content, but without injury toits papermaking characteristics.

2. A process for the production of a white high alphacellulose-containing fiber, which comprises the following sequence ofsteps, to wit: conditioning raw pulp for purification in an oxidizingliquor; treating said conditioned pulp with an alkaline liquor to eifecta removal of non-alpha cellulose constituents contained therein, therebyproducing a high alpha cellulose fiber; bleaching said fiber;

and retreating said bleached fiber with an alkaline liquor of a strengthto cause a solution of oxy-celluloses formed during bleaching

